1. Field of the Invention
This invention relates to photodiodes and, more particularly, to a silicon carbide photodiode which contains multiple epitaxially grown layers and exhibits high short-wavelength sensitivity, particularly in the ultraviolet spectrum, and very low reverse leakage current.
2. Description of the Prior Art
Currently, flame detectors find use in a wide variety of applications. Such applications include use in jet engines and gas turbines.
In particular, one such illustrative application involves detecting whether an afterburner in a jet engine is operating or not. In this application, a Geiger-Mueller tube has been and still is typically used as the detector. The tube is positioned to monitor an exhaust plume of the engine. Apart from the relatively large physical size of such tubes and the disadvantageous associated need for a high voltage transformer and ancillary circuitry, the performance of a Geiger-Mueller tube degrades with increasing temperature which, in turn, tends to limit the utility of these tubes with modern jet engines. Specifically, a Geiger-Mueller tube is a tube that has been filled with an ionizable gas. Optical emission produced by a jet engine flame, for example, causes a localized discharge to occur within the gas and eventually spread across the entire length of the anode contained in the tube. This discharge phenomenon causes the tube to produce an output pulse. For further details of the operation of these tubes, see, e.g., section 85 at page 10-42 of D. G. Fink et al., Electronic Engineers' Handbook, 3rd Edition, 1989, McGraw Hill, New York. As temperature of the tube rises, particularly above 200.degree. C. the heat experienced by the tube causes the gas to begin to self-ionize which, in turn, causes performance of the tube to begin degrading. Inasmuch as modern jet engines are being operated at increasingly higher temperatures, which also raise the temperature of the housing of the engine and that of the detector affixed thereto, flame detectors are now needed which can tolerate temperatures of 200.degree.-400.degree. C. Unfortunately, Geiger-Mueller tubes cannot perform adequately at these elevated temperatures. There is need, therefore, to replace the Geiger-Mueller tubes in jet engine flame detectors with appropriate detectors that will properly function at such elevated temperatures.
An optical detector that appears to exhibit excellent promise for use at such high temperatures is a silicon carbide photodiode. One such photodiode is shown and described in J. A. Edmond et al. U.S. Pat. No. 5,093,576, "High Sensitivity Ultraviolet Radiation Detector", issued Mar. 3, 1992.
Silicon carbide is a compound semiconductor that exists in a relatively large number of different crystalline forms, of which the 6H form (with "H" representing hexagonal crystalline packing) is the most readily available. Advantageously, the 6H form of silicon carbide exhibits a relatively wide band gap of approximately 3.0 electron volts. Such a wide band gap permits a 6H silicon carbide photodiode to possess excellent sensitivity to ultraviolet radiation, as well as low leakage current. In this regard, an ordinary silicon diode operating at 300.degree. C. possesses leakage current on the order of 10 mA/cm.sup.2, while a 6H silicon carbide diode operating at the same temperature exhibits leakage current on the order of 1-10 nA/cm.sup.2, which is advantageously some six orders of magnitude less. Furthermore, owing to the wide band gap, a 6H silicon carbide photodiode would be substantially, if not totally, transparent to both infrared and visible light. In this regard, such photodiode begins to exhibit optical sensitivity to applied light at wavelengths of approximately 400 nm and exhibits a peak response at approximately 270 nm. Accordingly, such detector could readily discern ultraviolet radiation that exists in the presence of a strong background of intense infrared and/or visible light. As such, one would expect that the detector would readily respond to the ultraviolet emission produced by a flame in a jet engine while effectively ignoring all the infrared and visible radiation emitted by the heated parts of the engine itself.
While the above benefits attainable from silicon carbide photodiodes would clearly make them superior in high temperature applications to other types of photodetectors, practical realization of silicon carbide photodiodes has been very problematic at best.
In particular, silicon carbide diodes are currently manufactured using either of two basic techniques: ion implantation or epitaxial growth. Through ion implantation, silicon carbide diodes have been fabricated to contain a very shallow n+/p junction (approximately 500 Angstroms deep) formed by nitrogen implantation in a p type conductivity epitaxial layer. See, for example, P. Glasow et al., "SiC-UV Photodetectors", SPIE 868, Optoelectronic Technologies for Remote Sensing from Space, pp. 40-45, 1987. Although the quantum efficiency of these diodes was found to be relatively high in a 200-400 nm wavelength region, ion implantation caused significant crystal damage. Unfortunately, this damage not only proved to be extremely difficult to anneal but, more importantly, also caused these diodes to exhibit excessively high reverse leakage current. For example, at 300.degree. C., ion implanted silicon carbide diodes exhibited leakage currents on the order of between 1-10 mA/cm.sup.2. Such high leakage currents are well in excess of what can be tolerated in a photodiode.
Given the drawbacks, particularly the excessively high leakage currents, associated with shallow junction silicon carbide diodes manufactured using ion implantation, epitaxial growth has emerged in the art as a favored process to produce silicon carbide diodes. However, as will now be seen, even this process, as least as it is currently applied in the art, can be improved upon as evidenced by the present invention.
Typically, as known in the art, a silicon carbide diode manufactured using epitaxial growth begins with either a 6H n or p type conductivity substrate. If an n type conductivity substrate is used, then a heavily doped p+ layer, of approximately 1 .mu.m in thickness, is epitaxially grown over the substrate. A lightly doped p type layer (i.e. a p- layer), between approximately 1 and 5 .mu.m in thickness and typically using aluminum as the dopant, is then grown over the p+ layer. Alternatively, if a p type substrate is used, then the p- layer is grown directly over the substrate. The thickness of the p- layer can be set to a value between 1 and 5 .mu.m depending upon the desired optical sensitivity of the diode. In particular, owing to the relatively low optical coefficient of absorption of silicon carbide, a relatively thick p- layer is used in order to increase the sensitivity of the resulting photodiode to long ultraviolet wavelengths, while this p- layer is made relatively thin--particularly if the diode is to be used as a flame detector--to decrease sensitivity of the diode to long ultraviolet wavelengths and thus decrease its sensitivity to solar radiation occurring between 300 and 400 nm. Next, to form a junction, an n+ layer, typically using nitrogen as the dopant, is epitaxially grown at a uniform thickness over the p- layer. A metallic contact is then formed on top of the n+ layer. A metallic contact is also made to the back side of the p type substrate or, if an n type substrate is being used, to an exposed portion of the top of the p+ layer.
In order to increase sensitivity of the resulting silicon carbide photodiode to short ultraviolet wavelengths, particular those that exist in jet fuel combustion, the heavily doped n+ epitaxial layer would need to be made quite thin, typically on the order of 1000 Angstroms or less. Unfortunately, in practice, placing a metallic contact onto such thin layer can cause so-called "contact spiking" to occur, resulting in excessively high diode leakage current. Specifically, silicon carbide crystals, unlike pure silicon, are not defect-free. In fact, silicon carbide crystals generally contain so-called micro-defects. Whenever a metallic layer is alloyed or sintered onto such a crystalline silicon carbide layer in order to produce a good ohmic contact, the metal seeks out and diffuses into the defects in the crystalline layer. Unfortunately, if the n+ layer is relatively thin, as is taught in the art to increase the short wavelength sensitivity of the resulting diode, then the metal can migrate through this layer. The very leaky diode caused by this "contact spiking" is unacceptable for use as an accurate flame detector.
Faced with this deficiency, it appears that a relatively thick n+ layer must be used to avoid contact spiking but at the expense of reducing the sensitivity at 280 nm and below. In fact, if the n+ layer is made too thick, then the short ultraviolet wavelength sensitivity, desirable for use in a flame detector for a jet engine, would be lost.
In view of this background, it is apparent that a need exists in the art for a silicon carbide photodiode which contains multiple epitaxially grown layers and exhibits both high short-wavelength sensitivity, particularly to short ultraviolet illumination, and very low reverse leakage current. Such photodiode should find ready acceptance as an accurate high temperature photodiode and, advantageously, as an accurate flame detector for use in a jet engine, gas turbine or other high temperature environment.